The invention relates to a method of preparing aliphatic polyisocyanates with uretdione, isocyanurate or iminooxadiazindione structures, the isocyanates thus prepared and polyurethane paints and coatings containing them.
Oligomerization of isocyanates is a long-known, generally accepted method of modifying low molecular weight isocyanates, which are usually difunctional, in order to obtain products with advantageous application properties for example in the paint and coating sector. These oligomeric isocyanates will be referred to generally as polyisocyanates in this specification (J. Prakt. Chem./Chem. Ztg 1994, 336, 185-200). Polyisocyanates based on aliphatic diisocyanates are normally used for light-resistant, non-yellowing paints and coatings. The term xe2x80x9caliphaticxe2x80x9d refers to the carbon atoms to which the NCO groups of the monomer are bonded, i.e. the compound molecule may perfectly well contain aromatic rings, which do not carry NCO groups.
One can distinguish between different products and processes according to the type of structure mainly formed from the previously free NCO groups in the respective oligomerization reaction.
Particularly important procedures are so-called dimerization to form uretdione structures of formula 1, described for example in DE-A 16 709 720 and so-called trimerization to form isocyanate structures of formula 2, described for example in EP-A 0 010 589. In addition to the last-mentioned trimers isomeric (i.e. also trimeric) products with an iminooxadiazindione structure of formula 3 can be obtained for example according to the teaching of EP-A 0 798 299. If this specification refers to both isomeric trimers, isocyanurates and iminooxadiazindiones, it will generally be speaking of trimers or trimerized compounds, otherwise the exact term will be used. 
R=difunctional substituent
Complete conversion of all monomeric diisocyanate molecules OCN-R-NCO in one reaction step would in practice lead to high molecular weight, extremely high-viscosity or gel-like products which would be useless in the paint and coatings sector, owing to further reaction of the NCO groups in ideal structures 1 to 3. In catalyzed preparation of polyisocyanates for paint the industrial procedure is therefore to convert only part of the monomer, to stop any further reaction by adding a catalyst poison (a xe2x80x9cstopperxe2x80x9d) then to separate the non-converted monomer. The aim is to have to separate the smallest possible proportion of non-converted monomer at the lowest possible viscosity of the low-monomer polyisocyanate paint resin, i.e. to obtain high conversion in the reaction accompanied by a high resin yield at the following processing stage with high-level properties of the polyisocyanate resins.
Dimers based on aliphatic diisocyanates have a far lower viscosity than trimers. However they have a strictly linear, i.e. NCO-difunctional structure regardless of the degree of conversion or the resin yield. Trimers on the other hand have the higher functionality required for a high crosslink-density in the polymer and consequent good stability properties thereof. Their viscosity increases very rapidly though with increasing conversion in the reaction. Compared with isomeric isocyanurates iminooxadiazindiones have far lower viscosity with the same NCO-functionality of the polyisocyanate resin (cf. Proc. of the XXIVth Fatipec Conference, Jun. 8-11, 1998, Interlaken, CH, vol. D, pp. D-136-137), though they do not reach the viscosity level of uretdiones.
In practice the products formed are not only those which give their names to the reaction (dimers in dimerization, trimers in trimerization) but nearly always the other reaction products simultaneously (trimers in dimerization, uretdiones in trimerization). However the content of these is low.
Thus trimers are always contained in the polyisocyanates accessible from the teaching of DE-A 16 70 720 (trialkylphosphine-catalyzed dimerization, cf. also comparative example 1). Their content can be increased somewhat with higher conversion (conversion-dependent selectivity) and by raising the temperature. However carbodiimides and secondary products thereof, particularly uretone imines are also formed increasingly in the latter case. The negative role of such products in the technology of aliphatic polyisocyanates and poor monomer stability in particular have been described elsewhere (cf. EP-A 798 299, p.4, line 42 to p.5, line 15). Their formation is thus undesirable in all cases and stands in the way of wide, hazard-free use of such polyisocyanates. The molar uretdione content of carbodiimide-free and uretone imine-free products prepared according to DE-A 16 70 720 is generally over 60%. In this specification the term xe2x80x9cmol %xe2x80x9d will, unless otherwise stated, always refer to the sum of structure-types formed by the modifying reaction (oligomerization) from the previously free NCO groups of the respective monomer. The molar content can be determined for example by NMR spectroscopic methods (cf. Examples).
Oligomerization of aliphatic diisocyanates with N-silyl compounds, described for example in EP-A 57 653, EP-A 89 297, EP-A 187 105, EP-A 197 864 and WO 99/07765 (cf. also comparative Example 2), is to a certain extent the counterpart of phosphine catalysis with the content of trimers and uretdiones reversed. One drawback here is that the selectivity of catalysis is highly dependent on conversionxe2x80x94the uretdione content of the polyisocyanates drops sharply as conversion increasesxe2x80x94another is that the iminooxadiazindione content of the resins is very low, always well below 5 mol %. According to the teaching of WO 99/07765 step-by-step modification is possible through thermally induced, i.e. non-catalyzed or rather self-catalyzed uretdione formation preceding the silylamine-catalyzed trimerization. Apart from the above-mentioned general drawbacks of silylamine catalysis this procedure however has the disadvantage that thermally induced uretdione formation is a slow process, leading to long, industrially unacceptable overall reaction times, particularly if there is an attempt to obtain a higher uretdione content. The molar uretdione content of the products of WO 99/07765 is not more than 30%.
In products accessible from the teaching of EP-A 798 299, i.e. trimerized compounds with a high content of iminooxadiazindione structures, the uretdione content is similarly low ( less than  less than 20 mol %) relative to the sum of isocyanurate and iminooxadiazindione structures. The selectivity of the reaction is also dependent on conversion and temperature.
Good methods for obtaining polyisocyanates for paints with the lowest possible viscosity and the highest possible NCO-functionality can be characterized as follows:
1. The trimerization reaction is interrupted at very low conversion rates as the higher-molecular type of compound with more than one isocyanurate ring per molecule, which is responsible for the increase in viscosity, is not yet very advanced then, or
2. polyisocyanates of the uretdione type are mixed with trimerized compound resins; here the uretdione component is so to speak the reactive diluent.
Yet both methods have specific disadvantages. Thus in the first method the resin yield is very small, giving rise not only to technical problems (separating a large quantity of monomer) but also to economic ones (space-time yield) and ecological drawbacks (energy consumption). When different paint resins are mixed, apart from the general drawback of having an additional step in the process it must be realized that viscosity will be increased by the build-up of molecular weight through the consecutive addition of further monomer molecules to the NCO groups of ideal structure 1 (polymer chain formation), even in the preparation of dimerized compounds with higher conversion; this is so even if the increase in viscosity is not as marked as with trimerized compounds (star-shaped progress of oligomer build-up). Hence dimerized resins which are suitable as reactive diluents are similarly produced with a relatively low resin yield.
It was an object of the invention to provide a method of preparing polyisocyanates based on aliphatic diisocyanates, which have a high content of uretdione structures in addition to trimer structures. They have to be prepared in one reaction stage, i.e. without stages preceding or following the catalyzed reaction such as physical mixing of different resins, pre-reaction tempering and others. The structural composition of the polyisocyanates, i.e., the molar ratio of uretdione, isocyanurate and iminooxadiazindione structures to each other, must be relatively independent of the conversion and be distinguished by low product viscosity with high NCO-functionality and high resin yield.
It has been found that the object is achieved according to the invention by the use of saline compounds from the range of de-protonized 1,2,3- and 1,2,4-triazoles as catalysts for oligomerizing monomeric aliphatic isocyanate.
The invention is based on the surprising observation that the effect of saline compounds from the range of 1,2,3- and 1,2,4-triazoles (triazolates) on aliphatic isocyanates, apart from producing isocyanurate structures, simultaneously leads firstly, to a considerable extent, to the formation of iminooxadiazindione structures isomeric to the latter, but secondly can form a high uretdione content in the products. What is particularly surprising is that the selectivity of the reaction, i.e. the molar ratio of the different types of structure to each other, is hardly dependent on the extent of monomer conversion.
Products with such a combination of different types of oligomer structure cannot be obtained by any single-stage state of the art isocyanate oligomerizing process. It is also novel to obtain a molar ratio of the different types of structure which is almost independent of conversion when the polyisocyanate has a high uretdione content.
Generally speaking it is hardly possible to predict the selectivity of isocyanate-oligomerizing catalysts. Here empirical tests are still required. Although neutral heterocycles carrying N13 Hxe2x80x94 or N-alkyl groups have already been introduced in polyisocyanate chemistry they are used almost exclusively in applications as blocking agents for NCO-groups (derivatives containing NH-groups, cf. EP-A 0 741 157) or as stabilisers to prevent UV-induced decomposition of paint films made from polyisocyanates, for example substituted benzotriazoles containing other OH groups in the molecule, for example DE-A 198 28 935, WO 99/67226 and literature quoted therein.
What is attempted here is not oligomerization of isocyanate groups but their thermally reversible deactivation to allow single-component processing or stabilisation respectively of the polyurethane plastic or paint. Oligomerization of isocyanate groups would even be a disadvantage in either case.
The invention relates to a method of preparing an aliphatic polyisocyanate by conversion of an aliphatic diisocyanate to a polyisocyanates in the presence of a catalyst, wherein the catalyst contains a saline compound having 5 to 97.1 wt. % of a 1,2,3- and/or a 1,2,4-triazolate (calculated as C2N3, molecular weight 66) in the anion. The invention also relates to polyisocyanates obtained by this method and the polyurethane plastics and coatings containing them.
Suitable aliphatic isocyanates include any regio and stereo-isomers of the following isocyanates: bis(isocyanate alkyl)ether, bis- and tris-(isocyanate alkyl)-benzenes, -toluenes and -xylenes, propane diisocyanates, butane diisocyanates, pentane diisocyanates, hexane diisocyanates (for example hexamethylene diisocyanate, HDI), heptane diisocyanates, octane diisocyanates, nonane diisocyanates (for example trimethyl-HDI, generally as a mixture of 2,4,4- and 2,2,4-isomers, TMDI) and triisocyanates (for example 4-isocyanate methyl-1,8-octane diisocyanate), decane di- and triisocyanates, undecane di- and triisocyanates, dodecane di- and triisocyanates, 1,3- and 1,4-bis(isocyanate methyl)cyclehaxane (H6XDI), 3-isocyanate methyl-3,5,5-trimethylcyclohexyl isocyanate (isophorone diisocyanate, IPDI), bis-(4-isocyanate cyclohexyl)methane (H12MDI) and bis(isocyanate methyl)norbornane (NBDI).
The starting materials for preparing the polyisocyanates obtainable by the method of the invention are in particular aliphatic diisocyanates which have 4 to 20 carbon atoms in the carbon network apart from the NCO groups, the NCO groups being bonded to primary aliphatic carbon atoms.
Preferred aliphatic diisocyanates include HDI, TMDI, 2-methylpentane-1,5-diisocyanate (MPDI), 1,3-and 1,4-bis(isocyanate methyl)cyclohexane (H6XDI, optionally as an isomeric mixture) and/or bis(isocyanate methyl)norbornane (NBDI optionally as an isomeric mixture).
Part-use of monofunctional isocyanates is also possible in special cases but is not preferred.
The production process for the initial isocyanates to be used in the method of the invention is not critical to carrying out the method, thus the initial isocyanates may be produced with or without using phosgene. Some of the catalysts used in the method of the invention are commercially available, for example as sodium salts. On the other hand they can be produced very easily, for example if counter-ions other than Na+ are to be used for the catalytically active anion. Details can be found in the examples.
The catalysts used are saline compounds containing in the anion triazolate structures of formulae (I) and/or (II) 
where
R1, R2, R3 and R4 independently represent hydrogen, fluorine, chlorine, bromine, nitro, a saturated or unsaturated aliphatic or cycloaliphatic radical, a substituted or unsubstituted aromatic or araliphatic radical having up to 20 carbon atoms and optionally up to 3 heteroatoms selected from the group consisting of oxygen, sulphur and nitrogen and which may optionally be substituted by halogen atoms or nitro groups,
and where
R3 and R4, combined and together with the carbon atoms of the 1,2,3-triazolate five-ring compound and optionally a further nitrogen atom or an oxygen atom, can form annellated rings with 3 to 6 carbon atoms.
The catalysts used are preferably saline compounds which contain in the anion triazolate structures of formula (I), where
R1 and R2 independently represent a hydrogen atom, a halogen atom from the fluorine, chlorine or bromine range or a nitro group, a saturated aliphatic or cycloaliphatic radical, a optionally substituted aromatic or araliphatic radical which contains up to 12 carbon atoms and optionally up to 3 heteroatoms from the oxygen, sulphur and nitrogen range and which may optionally be substituted by halogen atoms or nitro groups, and
saline compounds which contain in the anion triazolate structures of formula (II), where
R3 and R4 independently represent a hydrogen atom, a halogen atom from the fluorine, chlorine or bromine range or a nitro group, a saturated or unsaturated aliphatic or cycloaliphatic radical, a optionally substituted aromatic or araliphatic radical which contains up to 12 carbon atoms and optionally up to 3 heteroatoms from the oxygen, sulphur and nitrogen range and which may optionally be substituted by halogen atoms or nitro groups and, combined and together with the carbon atoms of the 1,2,3-triazolate five-ring compound and optionally a further nitrogen atom or an oxygen atom, can form annellated rings with 3 to 6 carbon atoms.
The optimum xe2x80x9cdesignxe2x80x9d of the anion in respect of catalytic activity, thermal stability and the selectivity of the reaction for the problem posed above may further be adapted to the isocyanate to the oligomerized or to the desired reaction conditions by appropriate substitution in the heterocyclic five-ring compound. Saline catalysts with a triazolate anion are generally suitable for preparing the polyisocyanates according to the invention, and in the neutral form of triazole at least one Zerewittinoff-active hydrogen atom bonded to a cyclic nitrogen atom has to be present. Some examples of these triazoles with NH groups on which the catalysts include 1,2,3- and 1,2,4-triazole and substituted derivatives, derived by replacing one or both C-bonded H atoms with optionally substituted (O, N, S, halogen) alkyl or aryl groups, for example 4-chloro-5-carbomethoxy-1,2,3-triazole, 4-chloro-5-cyano-1,2,3-triazole or 3,5-dibromotriazole. In the 1,2,3-triazoles finally the two adjacent C atoms may themselves again be part of an annellated ring system optionally containing further heteroatoms (O, N, S). Some examples of these are 1,2,3-benzotriazole and substituted 1,2,3-benzotriazoles such as 5-fluoro-1,2,3-benzotriazole, 5-trifluoromethyl-1,2,3-benzotriazole, 5-nitro-1,2,3-benzotriazole, 5-methoxy-1,2,3-benzotriazole, 5-chloro-1,2,3-benzotriazole, 5-tetrafluoroethoxy-1,2,3-benzotriazole, 5-trifluorothio-1,2,3-benzotriazole, 4,6-bis(trifluoromethyl)-1,2,3-benzotriazole, 4-trifluoromethoxy-5-chloro-1,2,3-benzotriazole and heteroaromatically annellated 1,2,3-triazoles such as isomeric pyridine triazoles for example 1H-1,2,3-triazol[4,5-b]pyridinexe2x80x94hereinafter referred to briefly as pyridine triazole- and azapurin. Salts of 1,2,4-triazole, 1,2,3-triazole, 1,2,3-benzotriazole and/or pyridine triazole are preferred as catalysts in the method of the invention.
The above-mentioned compounds are predominantly substances which are known from the literature. The synthesis of the fluorine-containing derivatives is described for example in DE-A 43 02 461. The cation for the catalysts used in the method of the invention may vary widely. If the catalyst or its secondary products formed in the course of deactivation are to be separated from the product after the oligomerization reaction, it may be advantageous to employ polar, highly charged counter-ions such as alkaline or alkaline earth cations. If the catalyst has to be distributed as homogeneously as possible in the isocyanate (mixture) used for the reaction and the polyisocyanate resin, lipophilic ones such as ammonium or phosphonium types can be chosen. The latter can for example be prepared without any problems simply by combining a sodium triazolate and an ammonium or phosphonium chloride, preferably in solvents which do not readily dissolve the sodium chloride formed, and bringing the mixture to the desired concentration and purity by filtration and subsequent reduction. During the last processing stage residues of initially dissolved sodium chloride are generally also precipitated and can be filtered off. Some examples of suitable ammonium or phosphonium chlorides are tetra-methyl, -ethyl, -propyl, -butyl, -hexyl and -octyl ammonium chloride but also ammonium salts which are substituted mixed, such as benzyl-trimethylammonium chloride or methyl-trialkylammonium chlorides where alkyl stands for straight-chain or branched C8 to C10 radicals (brand name for example Aliquat or Adogen) and tetra-ethyl, -propyl, -butyl, -hexyl and -octyl-phosphonium chloride, but also phosphonium salts which are substituted mixed, such as alkyl-triethyl, tributyl, trihexyl, trioctyl and/or tridodecylphosphonium chloride, where alkyl stands for straight-chain or branched C4 to C20 radicals (brand name for example Cyphos, such as Cyphos443, Cyphos3453, Cyphos3653 and others).
The catalyst concentrations employed when carrying out the method of the invention are between 5 ppm and 5%, preferably between 10 ppm and 1 wt. %, relative to the mass of the initial (di)isocyanate or (di)isocyanate mixture used and the mass of catalyst used.
The catalysts employed in the method of the invention may be used without solvent or in solution. The solvents may basically be any substances in which the catalyst can dissolve without decomposition and which do not react with isocyanates or react with them only to form secondary products that are common in polyurethane chemistry such as ureas, biurets, urethanes and allophanates. If catalyst solvents are employed they are preferably reactive compounds which react with the diisocyanates used as starting components to form secondary products that are common in polyurethane chemistry; hence these compounds need not be separated after the reaction. They include straight-chain or branched alcohols, optionally containing more than one OH group, with 1 to 20 carbon atoms and optionally other heteroatoms, preferably oxygen, in the molecule. Some examples are methanol, ethanol, 1- and 2-propanol, isomeric butanols, 2-ethylhexanol, 2-ethylhexane-1,3-diol, 1,3- and 1,4-butanediol and 1-methoxy-2-propanol. It is particularly advantageous that the above catalysts may be used even in very concentrated solution yet hardly cause any spontaneous over-curing in the starting material.
There are known methods of inhibiting further reaction when the desired stage has been reached (xe2x80x9cstoppingxe2x80x9d the reaction), such as removing the catalyst by extraction or filtrationxe2x80x94the latter optionally after adsorptive bonding to inert carrier materialsxe2x80x94making the catalyst system inactive by thermal deactivation and/or by adding (sub)stoichiometric quantities of acids or acid derivatives including for example benzoyl chloride, phthaloyl chloride, phosphinous, phosphonous and/or phosphorous acid, phosphinic, phosphonic and/or phosphoric acid, acid esters of the 6 last-mentioned acid types, sulphuric acid and its acid esters and/or sulphonic acids.
In a special embodiment of the method the polyisocyanates may be prepared in a continuous operation, i.e. in a pipe reactor.
The method of the invention can be carried out without any modifying stages preceding or following the catalyzed oligomerization reaction, such as thermal activation of the initial (di)isocyanate(s) before the catalyst is added or subsequent mixing of different resins.
Catalytic conversion according to the invention may take place within a very wide temperature range. Reaction temperatures above 0xc2x0 C. are normal; the preferred operating range is from 20 to 100xc2x0 C. and the more preferred one from 40 to 90xc2x0 C.
The products of the method according to the invention may be separated and purified by known processes such as thin-layer distillation, extraction, crystallization and/or molecular distillation. They are then in the form of colorless or only slightly colored liquids or solids.
Solid polyisocyanates are generally formed when cycloaliphatic diisocyanates are used as starting materials in the method of the invention. These solids are normally dissolved in one of the paint solvents listed below. The solutionsxe2x80x94at the same concentrationxe2x80x94have a lower viscosity than corresponding solutions of polyisocyanates of cycloaliphatic diisocyanates which predominantly contain isocyanurate groups.
The products prepared according to the invention are versatile starting materials for the production of polymers such as plastics, optionally foamed ones, and paints, coating materials, adhesives and additives.
These products, optionally in NCO-blocked form, are particularly appropriate for making single and dual-component polyurethane paints, since they have lower viscosity than polyisocyanates of the trimer type but otherwise equally good or improved properties. They may be used for this purpose either pure or combined with other known isocyanate derivatives such as uretdione, biuret, allophanate, isocyanurate, urethane or carbodiimide polyisocyanates in which the free NCO groups may optionally have been blocked.
The resultant plastics and coatings are extremely high-grade products with properties typical of prior art systems.
When used for example as cross-linking components in 2 component coatings the products according to the invention are generally combined with known OH and/or NH components from 2 component polyurethane systems, for example hydroxy-functional polyesters, polyacrylates, polycarbonates, polyethers, polyurethanes and polyfunctional amines. However they may equally be used as single components for example for making moisture-curing plastics and coatings.
Apart from the products according to the invention and any other binder components and paint solvents or paint solvent mixtures which may also be used, such as toluene, xylene, cyclohexane, chlorobenzene, butylacetate, ethylacetate, ethylglycolacetate, methoxypropylacetate, acetone, white spirit or higher substituted aromatics (trade names Solventnaphtha, Solvesso, Shellsol, Isopar, Nappar, Diasol), the coatings may contain further additives, for example wetting agents, flow control agents, anti-skinning agents, anti-foam agents, flatting agents, viscosity regulators, pigments, dyes, UV absorbers, catalysts and stabilizers to prevent thermal effects and oxidation.
The isocyanates prepared according to the invention may preferably be in the production of polyurethane plastics and coatings or as additives for inclusion in many different materials such as wood, plastic, leather, metal, paper, concrete, masonry, ceramics and textiles.